It is well known that, in the technology of synthesizing polypropylene, Ti/Mg catalyst systems are widely used in the synthesis processes of isotactic polypropylene due to their characteristics such as high efficiency, high stereospecificity (isotacticity) and so on. The catalysts used in the industrial production of polypropylene should meet two main requirements: one is that the catalyst should exhibit a relatively high activity in polymerization reaction, and another is that the resultant polymer should exhibit good integrated performance. The main properties of polypropylene include isotacticity of polymer, molecular weight distribution, particle morphology of polymer etc. Among these the particle morphology of polymer is particularly important in the industrial scale production of polypropylene.
The known patents of Ti/Mg catalyst systems improve typically the polymer particle morphology by effectively controlling particle morphology of catalyst via the optimization of components and synthesis process of the catalyst, wherein the selection and preparation of catalyst support are extremely important.
In order to improve catalytic activity, many patents utilize various physical or chemical processes to prepare active magnesium chloride support, and then load the support with a transition metal titanium compound and an electron-donor compound to form an active center of the catalyst. For example, in U.S. Pat. No. 4,784,983, anhydrous magnesium chloride is firstly dissolved in a solvent system to form a solution, then titanium tetrachloride as active component and a polybasic carboxylic acid ester as electron-donor are added to the solution, and the temperature of the resultant solution is elevated in the presence of phthalate anhydride as co-precipitator so that solid catalyst component containing active center is precipitated. When used in the polymerization reaction of propylene, the catalyst exhibits characteristics such as high activity and high isotacticity. However, since the catalyst particles are prepared by precipitation process, the particle size of the catalyst is relatively small and the catalyst particle morphology is relatively difficult to be stably controlled by temperature programming. In addition, for facilitating the precipitation of solids, co-precipitator and a large amount of titanium tetrachloride, which increase the cost of catalyst and cause environment pollution problem, are required.
In addition, many known patents load magnesium chloride onto porous inorganic oxide support such as silica etc. to obtain a composite support of magnesium chloride and silica, then the composite support is treated with titanium halide and electron-donor compound to finally obtain a catalyst component for olefin polymerization. For example, GB2028347 discloses a process for preparing a catalyst component loaded onto a porous inorganic oxide support, comprising impregnating a silica support with a magnesium chloride solution, then evaporating the solvent to obtain a solid product, then reacting the solid product with a transition metal compound, especially a titanium compound. For another example, U.S. Pat. No. 4,913,995 discloses a technique for preparing a high performance polypropylene catalyst using silica as support, comprising dispersing a porous silica support containing surface hydroxyl groups into a solution of magnesium chloride in tetrahydrofuran, drying the resultant suspension to obtain a composite support MgCl2/SiO2, then treating said composite support with titanium tetrachloride and electron-donor compound to finally obtain a catalyst product. However, when the catalyst prepared from a support obtained via magnesium chloride solution impregnating process is used in the polymerization of propylene, the polymerization activity is not satisfying. This may be attributed to the fact that this impregnating process essentially utilizes the particle morphology of silica support itself to control the particle morphology of the final catalyst, while the particle size of porous silica is relatively large with average particle size being typically about 50 μm, so that the amount of active component loaded on silica is restricted, resulting in lower activity of the final catalyst.
Further, some known patents, such as CN1091748A, disclose a process comprising preparing a spherical support from a magnesium chloride-alcohol adduct and then loading a transition metal titanium compound and an electron-donor compound thereon. The polypropylene synthesized with this type of catalysts has a better polymer particle morphology, generally in spherical shape. However, since such spherical catalysts have relatively large particle size, they may readily be broken during the polymerization of propylene, and this is disadvantageous in the industrial scale production.
Thus, there still need a catalyst, which, when used in the polymerization of propylene, not only exhibits a relatively high catalytic activity and stereospecificity, but also can synthesize a polymer having a better particle morphology.
The present invention utilizes magnesium halide and silica as composite support, and improves the particle morphology of the catalyst by regulating the ratio of magnesium halide to silica. Further, the purpose of stabilizing the rate of catalytic polymerization reaction and improving the particle morphology of polymer so as to meet the requirements on catalyst performance of various polymerization processes can be achieved through the combination of the supports of the catalyst. In the meantime, when used in the polymerization of propylene, the catalyst exhibits relatively high polymerization activity and high stereospecificity.